1. Field of the Invention
The present invention generally relates to a color-image forming device that forms a color image by sequentially superimposing toner images of respective color components. In particular, the present invention relates to a color-image forming device, by means of which the timing for starting color-drift correction and the timing for canceling color-drift correction can be controlled.
2. Description of the Related Art
Among various color-image forming devices for color image outputs, a color-image forming device that utilizes the electrophotographic process forms the color image using, for example, toners of four different colors, such as Yellow, Magenta, Cyan, and blacK. Unless high position precision is provided when superimposing the toner images of the four colors, color drift, which is the artifact observed on the final color image, may be generated due to the positional deviation of each toner images, and thus the image quality may be degraded. Accordingly, in such a color-image forming device, a color-drift correction is executed in order to correct the color drift.
Specifically, in the color-image forming device, image-forming parts are provided for the four colors. Each image-forming parts is provided with a photoconductor, and an electric charging part, an exposure part, a developing part, and a cleaner, all of which are provided around the photoconductor. The surface of the photoconductor is uniformly and electrically charged by the electric charging part, the electrically charged surface is exposed to the laser beam emitted from the exposure part, and an electrostatic latent image is formed on the photoconductor. The latent image is developed by the developing part.
Meanwhile, a recording medium is provided to a first image-forming part (yellow), where the toner image on the photoconductor is transferred onto the recording medium. The recording medium is sequentially provided to other image-forming parts corresponding to other colors and at the respective image-forming parts, the toner images of respective colors are transferred onto the recording medium. When the transfer of image is completed, unnecessary toner left on the surface of the photoconductor is cleaned by the cleaner and is prepared for the next image formation. The recording medium having the toner images of respective colors transferred thereon is provided to a fixing part, where the toner is fixed onto the recording medium, and the final image is output.
In such a color-image forming device, when the positional deviation from the predetermined transfer position generates during the transferring of the toner images of respective colors onto the recording medium, the color drift generates due to misalignment of the toner images and therefore, the image quality of the color image to be formed degrades significantly.
As for the cause of the positional deviation of the toner images, sub-scanning registration deviation due to difference between the axes of photoconductors provided in the respective image-forming parts, inclination deviation due to misalignment of inclination in the main-scanning direction of the photoconductor and inclination of the optic systems, main-scanning registration deviation due to deviation of writing position of the images, and magnification deviation due to the difference in the length of scanning lines for respective colors may be considered.
In order to cope with such positional deviation of the toner images, various positional deviation adjustment methods are employed conventionally. For the sub-scanning registration deviation and the main-scanning registration deviation, a method of adjusting the scanning timing of the writing of the latent image by the exposure part may be employed. For the inclination deviation, a method of adjusting the inclination of a reflection mirror provided on the light path of the image-forming part may be employed. For the magnification deviation, a method of changing writing clocks of the writing of the latent image or a method of displacing the reflection mirror may be employed.
Further, as shown in FIG. 1, for example, three sensors 100 may be arranged in the main-scanning direction orthogonal to the feeding direction (rotational direction and sub-scanning direction) of a transfer belt 200. The number of sensors is arbitrary. At a plurality of positions on the surface of the transfer belt 200, which positions are to be read by the sensors, a toner mark (pattern) 300 (see also, FIG. 2) for position detection is formed by respective image-forming parts. Each sensor reads the toner mark, and the positional deviation of the toner image is detected based on the output from the sensors. Depending on the condition of the positional deviation of the toner images detected based on the output, the positions of the images formed by the image-forming parts on the photoconductor are adjusted so as to correct the color drift generated due to the positional deviation of the toner images. For the details of the above-mentioned. technique, see for example, Japanese Laid-Out Application No. 2003-186278.
For example, when the automatic color-drift correction is executed, a mark for color-drift correction is written onto an intermediate transfer medium, the written marks are detected by a sensor, and the writing position color-drift correction is electrically or mechanically executed depending on the detected result. (See for example, Japanese Laid-Open Patent Application No. 2000-305340.)
However, a single operation of such a color-drift correction in a single color-image forming device is not satisfactory. Rather, such a color-drift correction is to be executed according to various condition changes including, for example, ambient atmospheric changes such as temperature and relative humidity, variation per hour of the color-image forming device, exchange timing of toners or photoconductors, etc. In other words, various factors such as mentioned above may lead to the generation of such color drift.
In general, the color-drift correction is automatically executed in response to the condition changes such as mentioned above. However, the color-drift correction needs a predetermined amount of time (ranging from a few seconds to a few minutes) and therefore, there arises a problem in that the color-image forming device is not ready for use until the color-drift correction is completed even when a user of the device wishes to use it right away.
In addition, in the case when the color-image forming device has not been used for a predetermined period and when such a device is given a task (for example, for printing out), there arises a problem in that the device first begins the color-drift correction operation and thus the printing output cannot be quickly obtained.